The two long-term objectives of the current proposal are: 1. Investigate the neural bases and cellular mechanisms of behavioral plasticity afforded through the expression of multiple co-transmitters; 2. Prepare the candidate for a successful research career in the field of mental health. While I am already committed to such a career, I do not yet possess the broad ranging technical expertise necessary to independently undertake a multi-disciplinary, integrated investigation in my chosen field. Therefore, I propose to continue my research into the neural correlate of behavioral plasticity under the direction of a technically expert sponsor, Dr. K. R. Weiss. Dr. Weiss used the relatively, simple invertebrate model system called the marine snail Aplysia, to make considerable progress in understanding the role of peptide co-transmitters in the modulation of behavior. While much is known of the postsynaptic mechanisms of action of these peptide co-transmitters, less is understood about their pre-synaptic role(s). I suggest that peptide cotransmitters may act pre-synaptically to regulate the ratio of conventional and peptide transmitters released, and further, that the expression of multiple peptide cotransmitters affords the range of modulation of synaptic efficacy necessary to accommodate changing behavioral demands. In order to test these hypotheses, I have developed a neuromuscular preparation from Aplysia to directly measure the release of conventional and peptide transmitters, while simultaneously monitoring evoked EJPs and muscle contractions. In the current proposal, the behavioral firing patterns of the motor neurons involved will first be determined in freely moving animals. These stimulation paradigms will be used to determine the physiological range of conventional and peptide transmitter release in the reduced preparation. Then, the peptidergic modulation of conventional and peptide transmitter release and evoked responses will be measured. To complete the project, experiments utilizing specific blocking agents, voltage clamp and electron microscopy will elucidate the presynaptic site of action of the peptide co-transmitters. Apart from its scientific rationale, I propose this logically developing sequence of research, so that it will naturally involve my technical training as it becomes necessary for the research. In this way, my wish to answer the scientific questions, will become the motivation and goal of the technical training and my ability to answer them successfully on the validation of its adequacy. In view of the widespread presence of co-transmission and the pivotal role that regulation of synaptic transmission may play in various forms of behavioral plasticity including learning, motivation and development and in their pathologies, an understanding of this phenonenom in an animal such as Aplysia will contribute to the amelioration of human disorders in affect, cognition and movement.